1. Field of the Invention
The invention relates to an apparatus for generating laser radiation at a frequency that is multiplied as compared with a base frequency, having an optical resonator, in which input laser radiation circulates resonantly at the base frequency, and having at least one conversion element through which the input laser radiation that circulates in the optical resonator radiates and which converts this radiation, at least in part, into output laser radiation at the multiplied frequency. Furthermore, the invention relates to a system for generating laser radiation.
2. Description of the Related Art
The basic principle of frequency multiplication using non-linear optics is sufficiently known. In this connection, the laser radiation present at a base frequency (input laser radiation) is converted to laser radiation (output laser radiation), using a non-linear optical medium as a conversion element, at a frequency that is a whole-number multiple of the base frequency. A prerequisite for this conversion is that the non-linear optical medium of the conversion element has a susceptibility of a higher order. The frequency conversion based on this principle finds broad application. Frequently, solid-body lasers that can be produced in simple and cost-advantageous manner and emit in the infrared spectral range are used, in order to generate laser radiation in the visible or ultraviolet spectral range by means of frequency multiplication. Suitable non-linear optical media are commercially available in the form of crystals, for example.
The efficiency of the frequency conversion is greatly dependent on the intensity of the input laser radiation. In order to obtain the most intensive output laser radiation possible at the multiplied frequency, an optical resonator is therefore frequently used to super-elevate the intensity of the input laser radiation. The conversion element is then situated within the resonator, i.e. the input laser radiation that circulates in the resonator, i.e. is resonantly super-elevated, radiates through the non-linear medium.
A disadvantage is the power and mode stability in the generation of frequency-multiplied laser radiation, which is difficult to achieve, using a conversion element situated in the resonator. Already because of the non-linear relationship between the intensity of the input laser radiation and the efficiency of the frequency conversion, power oscillations and competing resonator modes can occur, which are difficult to stabilize.
At higher power, a further difficulty is that the absorption of the laser radiation in the optical resonator leads to a shift in the resonance frequencies due to thermal effects. In the non-linear conversion element, a significant part of the circulating laser radiation is absorbed by the crystal at high power, and is thereby converted to output radiation, for one thing, and to heat, for another. As a consequence, the temperature of the non-linear medium changes. Because of thermal expansion, the geometric length of the non-linear medium changes. Furthermore, the index of refraction of the non-linear medium is temperature-dependent. In total, the optical path length of the laser radiation radiating through the conversion element therefore varies in temperature-dependent manner. The optical path length in the conversion element determines the efficiency of the frequency conversion. Consequently, the thermal variation of the optical path length in turn results in a variation of the absorption of the input laser radiation. This variation makes resonant frequency multiplication at high power practically impossible to stabilize.
The phenomena described above will be explained below using FIGS. 1 and 2. FIGS. 1a and 2a each show an error signal E of a Pound-Drever-Hall regulator (PDH regulator) at a power of the input laser radiation circulating in an optical resonator of 1 W (FIG. 1) or 40 W (FIG. 2), for example. In the diagrams of FIGS. 1a and 2a, the error signal E is shown as a function of the frequency f, in each instance. FIGS. 1b and 2b each show the resonance curve of the optical resonator (also referred to as an Airy function). What is shown is the intensity I of the circulating laser radiation as a function of the laser frequency f. As FIG. 1 shows, the curves are symmetrical at low circulating power, and a clear relationship exists between error signal E or intensity I and frequency f. Stabilization of the resonator is possible by means of suitable regulation. The high circulating power and the thermally induced variation in the absorption of the input laser radiation in the non-linear conversion element situated in the resonator, caused by this power, brings about the distortion in the Airy function that can be seen in FIG. 2b. The normally symmetrical resonance curve is inclined at a strong slant. The curve shows that no clear relationship exists between intensity I and frequency f. At a given laser frequency f, the intensity I of the laser radiation can assume up to three different values. The system jumps chaotically between these simultaneously allowed states of the resonator, at the slightest interference. Accordingly, stable operation is not possible. This situation is shown by the diagram in FIG. 2a. The error signal E of the PDH regulator can assume up to three different values at one laser frequency. The signal jumps chaotically between these values, so that the PDH regulator is not able to stabilize the optical resonator.
The diagrams of FIGS. 1 and 2 show simulation calculations. In this connection, the starting point was a power of 1 W or 40 W, respectively, circulating in the resonator, as has already been mentioned. A β-barium borate crystal having a length of 20 mm was assumed to be the conversion element. At the same time, a typical absorption of 30 ppm per centimeter was assumed in the crystal. The finesse of the resonator amounts to 250 in the simulation calculation.
The instabilities of optical resonators described above, induced by temperature dependence, in which resonators absorption occurs, are known from the state of the art in a different connection (see, for example, Journal of the Optical Society of America B, Vol. 13, Number 9, pages 2041 ff).